1. Field of the Invention
The present invention relates to a particle beam system using a particle beam, such as an electron beam and, more particularly, to techniques for preventing the spread of contamination to the inside of an optical column mounted inside the particle beam system.
2. Description of Related Art
A specimen inspection instrument (particle beam system) has been developed which irradiates a specimen containing a liquid held on a film with a primary beam (particle beam), such as an electron beam, via the film and detects a secondary signal, such as backscattered, electrons, emanating from the specimen in response to the irradiation.
Some particle beam systems have capabilities of a scanning electron microscope (SEM) which irradiates a specimen with an electron beam scanning the specimen, the specimen being held on the upper surface of a film exposed to the atmosphere. The beam is emitted from an optical column (i.e., primary beam irradiation source) located below the film.
In this case, the electron beam reaches the specimen via the film. The upper side of the film on which the specimen is held is in an atmospheric-pressure ambient. The opposite surface (i.e., the lower surface of the film hit by the electron beam) is exposed to a vacuum ambient.
Therefore, a pressure difference is produced between the ambient on the upper side of the film on which the specimen is held and the ambient in contact with the lower surface of the film during inspection or observation of the specimen. An unwanted external force is applied to the film due to the pressure difference. In some cases, the film is damaged by the external force. If the film is damaged, the liquid in the specimen enters the optical column via the damaged portion of the film, the column being located in the vacuum ambient. As a result, the inside of the optical column will be contaminated with the liquid.
In a conventional technique, the aforementioned contamination of the inside of the optical column is prevented by mounting a partitioning valve (open-close valve) for partitioning off the space between the film and the optical column within the vacuum chamber interposed between the film and the optical column (see JP-A-2007-292702).
In the structure disclosed in JP-A-2007-292702, the optical column constituting the primary beam irradiation source emits an electron beam upward. That is, the column is of the inverted type. Damage to the film is detected by detecting a deterioration of the vacuum inside the vacuum chamber by a vacuum gauge. If the damage to the film is detected, the partitioning valve is immediately closed.
In practice, however, at the instant when the damage to the film is detected, intrusion of the liquid into the vacuum chamber and optical column has already produced adverse effects. It is difficult to avoid the effects using the above-described structure.
If the partitioning valve is moved mechanically at high speed (i.e., opened or closed quickly), the liquid pushed out by the pressure difference between the vacuum and the atmospheric pressure moves at a very high speed. Satisfactory results cannot be obtained unless the partitioning valve is actuated in a time on the order of nanoseconds, which cannot be easily achieved in practice using a mechanical structure.
The partitioning valve is usefully closed when the ambient on the lower side of a film equipped on a specimen holder is evacuated after a specimen holder consisting of a laboratory dish has been placed on the system.
That is, when (a) the film or a film support member located close to the film has deteriorated in pressure-withstanding performance, (b) when the bonded part between the material of the laboratory dish and the film support member has deteriorated in pressure-withstanding performance, or (c) when the tip of a manipulator has damaged the film while the specimen is being manipulated by the manipulator prior to SEM observation, the partitioning valve functions to protect the column.
In the specimen inspection system having the SEM consisting of the aforementioned inverted electron optical microscope, sufficient and effective countermeasures have not been taken at present against the spread of contamination of the inside of the optical column when the film holding the specimen is damaged.
There are three countermeasures against the problem. Countermeasure (1) is to reduce the frequency or probability at which the film is damaged. Countermeasure (2) is to reduce the effects of contamination of the optical column. Countermeasure (3) consists of instantly detecting generation of a damage to the film and quickly implementing a corresponding countermeasure against the contamination.
The countermeasure (1) consists of increasing the strength of the film, for example, by improving the quality of the film or giving a warning or caution if ultraviolet radiation is emitted to thereby increase the possibility of damage to the film or if electron beam irradiation is performed for a long time during SEM observation.
The countermeasure (2) consists of operating a partitioning valve mounted between the front end of an SEM column and a film during an interval starting with the beginning of vacuum pumping of the ambient on the lower side of the film and ending immediately before an SEM observation, thus reducing contamination of the inside of the column.
The countermeasure (3) consists of instantly detecting generation of a damage to the film and operating the shutter mounted between the front end of the SEM column and the film, thus suppressing contamination instantly.
With respect to the countermeasure (1), a high-quality film that is not easily broken has not been fabricated successfully.
With respect to the countermeasures (2) and (3), the opening and closing action of the shutter or partitioning valve is a mechanical operation. An operation completed in a time of the nanosecond order, which can cope with the velocity of motion of the liquid pushed out by the pressure difference between vacuum and atmospheric pressure, cannot be achieved whatever short the operation time is made.